License for Open-Source Software w/ Plugins?

ThiagoHP asks: "I have developed MultiMAD, a rapid application development and prototyping for for mobile devices in my master's course, and now I want to share it with the community. It's written in Java and it has a plugin architecture in order to support different mobile device platforms (WAP, J2ME, PalmOS, etc). I want to give the freedom to anybody write their plugins, even closed-source, as long as the tool itself is not modified for them to run. This implies that GPL cannot be used. At the same time, I do not want any closed-source tool based in MultiMAD code, so licenses such as the Apache one cannot be used. Am I right in my assumptions regarding licenses? What license do you suggest?"